The present invention relates to a fuel cell system including a fuel cell and a secondary battery and an electronic device having such a fuel cell system.
Traditionally, since a fuel cell has high power generation efficiency and does not exhaust harmful substances, it is practically used as an industrial or household power generating device or a power source of a satellite, a space ship, or the like. Further, in recent years, development of a fuel cell as a power source for a vehicle such as a car, a bus, or a truck is expanding. Such a fuel cell is classified into an alkaline aqueous solution type, a phosphoric acid type, a molten carbonate type, a solid oxide type, a direct methanol type, and the like. In particular, a direct methanol fuel cell (DMFC) realizes high energy density by using methanol as a fuel hydrogen source and also realizes miniaturization because a reformer is unnecessary. Therefore, such a DMFC is being studied so as to be used as a small portable fuel cell.
In a DMFC, an MEA (Membrane Electrode Assembly) as a unit cell obtained by sandwiching a solid polymer electrolyte membrane by two electrodes, and integrating and bonding them is used. When one of gas diffusion electrodes is set as a fuel electrode (anode) and methanol is supplied as fuel to the surface of the electrode, the methanol is degraded, and hydrogen ions (protons) and electrons are generated. The hydrogen ions pass through the solid polymer electrolyte membrane. Moreover, when the other gas diffusion electrode is set as an oxygen electrode (cathode) and air as an oxidant gas is supplied to the surface of the electrode, oxygen in the air and the above hydrogen ions and electrons are combined, and water is generated. By such an electrochemical reaction, electromotive force is generated from the DMFC.
Here, in an active fuel cell capable of adjusting the supply amount of the fuel, predetermined time is necessary since fuel supply starts until a steady power generation state is obtained. This is because it takes time for rise in catalyst temperature, moistening of an electrolyte film, and the like. Therefore, since a fuel cell has such characteristics, it is very difficult to cause an electronic device whose power demand dynamically changes to directly operate by a single fuel cell.
Then, to address such an issue, a method of connecting an output of a fuel cell in parallel with a secondary battery such as a lithium ion battery is proposed (for example, patent document 1). With such a configuration, a sharp change in the power demand is addressed by charging/discharging of the secondary battery. Consequently, the fuel cell itself may start slowly and perform only steady power generation. Therefore, by such a hybrid configuration of a fuel cell and a secondary battery, an electronic device whose power demand changes sharply operates stably.